1. Field of the Invention
The present invention relates to a method for forming an etching mask for a semiconductor integrated circuit by a photolithographic technology, and more particularly to an etching method by which to appropriately remove an antireflective coating formed under an etching-mask material to prevent reflected light when photographically patterning a resist layer to produce an etching mask.
2. Related Art
In the manufacture of a semiconductor device, a photolithographic technology is used. According to the photolithographic technology, a resist layer made of a photoresist material is formed on a layer to be processed, such as a metal layer, on top of a semiconductor substrate. After selectively exposed and then developed, unnecessary portions are removed from the resist layer, by which a resist mask is formed with a desired transfer pattern.
Through this resist mask as an etching mask, a metal layer for example, is etched by selective etching, and a wiring pattern corresponding to the desired pattern is formed by the remaining portion of the metal layer.
Meanwhile, with the growing density of the integrated circuits, in exposure of the photoresist, the deterioration of resolution by reflected light from the processing-object layer under the photoresist is becoming an increasingly serious problem.
To prevent the exposing rays from being reflected by the layer being processed in the exposure of the photoresist, a bottom antireflective coating (BARC) to absorb the light for exposure of the photoresist is disposed between the layer being processed and the photoresist, so that a decrease in the resolution caused by the reflected light is prevented.
However, after the photoresist is exposed and developed, when the processing layer is etched by using the bottom antireflective coating that covers the processing layer as an etching mask, the bottom antireflective coating needs to be removed without causing damage to the etching mask. Because the bottom antireflective coating is made of an organic material as the etching mask is, it is not easy to securely remove a portion of the bottom antireflective coating not covered by the etching mask without causing a substantial deformation in the etching mask.
JP-A-10-199868 discloses an etching method using a plasma etching unit. According to prior art described in the above publication, it is necessary to perform a first etching process that uses a mixed gas of oxygen, hydrogen bromide, carbon dioxide, and argon as an etching gas, and a second etching process that uses the same mixed gas as in the first, main etching process as an etching gas but uses the plasma etching unit in different operation conditions from those in the first etching process.
For the reason mentioned above, in the prior art, it is necessary to change over the operating conditions of the plasma etching unit.
An object of the present invention is to provide a novel etching method capable of appropriately remove unnecessary portions of a bottom antireflective coating not covered by photoresist without causing a substantial deformation to the photoresist as an etching mask and without changing over the operating conditions of the plasma etching unit.
Another object of the present invention is to provide an etching method capable of removing unnecessary portions of the bottom antireflective coating by desired etching characteristics by selecting operating conditions of the plasma etching unit.
The present invention adopts the following configuration to achieve the above object.
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According to the present invention, there is provided an etching method for, by a plasma etching unit, removing portions of a bottom antireflective coating not covered by an etching mask made of a photoresist material and having a desired shape for performing a selective etching process on a process-object film on a semiconductor substrate, the bottom antireflective coating being located between the process-object film and the photoresist material to prevent exposing rays from reflecting off the surface of the process-object film and traveling back up through the photoresist material when the photoresist material for forming the etching mask is selectively exposed to the rays to create the etching mask, comprising:
obtaining an ethyl alcohol addition ratio in a mixed gas not exhibiting isotropic etching characteristics to the etching mask by using a mixed gas of ethyl alcohol, oxygen and nitrogen as an etching gas in the plasma etching unit; and performing the etching process in the etching unit by using the etching gas, including ethyl alcohol, supplied at an addition ratio determined based on the obtained ethyl alcohol addition ratio to thereby remove the portions of the bottom antireflective coating not covered by the etching mask.
According to another present invention, there is provided an etching method for etching an antireflective film located under an etching mask made of a photoresist material, the method comprising: forming a first film over a semiconductor substrate, the first film being to be etched; forming the antireflective film on the first film; forming the etching mask on the antireflective film; removing portions of the etching mask to expose portions of the antireflective film; obtaining an ethyl alcohol addition ratio in a mixed gas not exhibiting isotropic etching characteristics to said etching mask by using mixed gas of ethyl alcohol, oxygen and nitrogen as an etching gas; and etching the exposed portions of the antireflective film by using said etching gas, including ethyl alcohol, supplied at an addition ratio determined based on said obtained ethyl alcohol addition ratio.
According to another present invention, there is provided a method for fabricating a semiconductor device, the method comprising: forming a first film over a semiconductor substrate, the first film being to be etched; forming an antireflective film on the first film; forming an etching mask made of a photoresist material on the antireflective film; removing portions of the etching mask to expose portions of the antireflective film; obtaining an ethyl alcohol addition ratio in a mixed gas not exhibiting isotropic etching characteristics to said etching mask by using mixed gas of ethyl alcohol, oxygen and nitrogen as an etching gas; etching the exposed portions of the antireflective film to expose portions of the first film by using said etching gas, including ethyl alcohol, supplied at an addition ratio determined based on said obtained ethyl alcohol addition ratio; and etching the exposed portions of the first film.
In the above-mentioned method according to the present invention, to remove unnecessary portions of the antireflective coating under the etching mask of a photoresist material on the bottom antireflective coating, a plasma etching unit same as those which have conventionally been used is used. In this plasma etching unit, as an etching gas, a mixed gas of ethyl alcohol, oxygen and nitrogen is used.
In the etching process using the above-mentioned etching unit, an ethyl alcohol addition ratio is obtained which does not show the above-mentioned mixed etching gas""s isotropic etching characteristics to the etching mask. Theoretically, it is possible to remove the portions of the bottom antireflective coating which are not covered by the etching mask by performing the etching process by using the mixed gas containing ethyl alcohol supplied at the obtained addition ratio as mentioned above, in other words, by etching without directionality in the etching characteristics, more specifically, by etching with an isotropic etching rate of zero, in which case horizontal etching, or side etching, of the etching mask itself is prevented, with the result that the deformation of the etching mask in its horizontal direction can be suppressed to a minimum.
However, in the experiment, the inventor could observe an optimum etching effect that the deformation of the etching mask in its horizontal direction was minimum at an ethyl alcohol addition ratio that did not show isotropic etching characteristics to the etching mask, more specifically, at an ethyl alcohol addition ratio slightly larger than the ethyl alcohol addition ratio at which the isotropic etching rate was zero.
In one case, a mixed gas was used which contained ethyl alcohol supplied at an addition ratio 5% larger than the addition ratio at which the isotropic etching rate of the etching mask was zero. The ethyl alcohol addition ratio can be selected suitably based on the ethyl alcohol addition ratio that makes the isotropic etching rate zero.
For an ethyl alcohol addition ratio at which the isotropic etching rate of the etching mask is zero, the value of an ethyl alcohol addition ratio is adopted when the etching rate of the etching mask is zero under operating conditions that the high-frequency power source voltage of the plasma etching unit is 0 volt. The etching rate could be a negative value, and when it occurs, this means that polymer is deposited on the etching mask, which will be discussed later. Therefore, if this phenomenon is used positively, it is possible to increase the widths of the mask patterns by suitably changing the addition ratio based on the above-mentioned addition ratio that does not show the isotropic etching characteristics.
The photoresist material and the bottom antireflective coating may be formed by organic materials, and the process-object film may be formed by a silicide made of metal material such as tungsten, a silicon material or a compound of them.
An example of flow rates of a mixed gas of the etching unit is 35 ccm, 35 ccm and 30 ccm for oxygen, nitrogen and ethyl alcohol, respectively.